Generally, it is old to provide a subsurface well safety valve for use in a well for shutting off flow of well fluids through the well tubing. U.S. Pat. Nos. 3,782,416; 3,786,865; and 3,799,258 disclose such safety valves in which the valve is biased to an open position and is closed by a piston in response to fluid applied from the well surface. However, the means biasing the valve to a closed position must overcome the hydrostatic head in the hydraulic control line to the piston. Because the hydrostatic force increases with depth, present-day piston actuated safety valves are limited in their depth of operation. For example, present-day spring closed valves are unable to function at depths greater than approximately 700 to 800 feet. Furthermore, the present-day pistons annularly surround the tubular member and are of a greater diameter than the valve passageway and increase the cost and complexity of manufacture.
The present invention is directed to an improved piston-actuated subsurface well safety valve in which the hydrostatic forces acting on the piston are reduced thereby allowing the safety valve to be utilized at much greater depths in the well and in which the cost and ease of manufacture are reduced.